Mwt architecture for thin si solar cells

ABSTRACT

Methods of fabricating metal wrap through solar cells and modules for thin silicon solar cells, including epitaxial silicon solar cells, are described. These metal wrap through solar cells have a planar back contact geometry for the base and emitter contacts. Fabrication of a metal wrap through solar cell may comprise: providing a photovoltaic device attached at the emitter side of the device to a solar glass by an encapsulant, the device including busbars on the device emitter; forming vias through the device base and emitter, the vias terminating in the busbars; depositing a conformal dielectric film over the surface of the vias and the back surface of the base; removing portions of the conformal dielectric film from the ends of the vias for exposing the busbars and from field areas of the base; and forming separate electrical contacts to the busbars and the field areas on the back surface of the solar cell. The solar cells may comprise epitaxially deposited silicon and may include an epitaxially deposited back surface field.

This application claims the benefit of U.S. Provisional Application Ser.No. 61/401,400 filed Aug. 11, 2010 and U.S. Provisional Application Ser.No. 61/454,363 filed Mar. 18, 2011, both incorporated by reference intheir entirety herein.

FIELD OF THE INVENTION

The present invention relates generally to solar cells, and moreparticularly to metal wrap through (MWT) thin epitaxial silicon solarcells.

BACKGROUND OF THE INVENTION

The simplest solar cells have contacts on the front and rear surfaces tocollect the negative and positive charge carriers. However thescreen-printed metal comprising the front-side contacts blocks asignificant area from receiving sunlight, often referred to as‘shadowing’. Some newer architectures have been proposed to addressthis. One such example of such devices are metal wrap through (MWT)devices, where the thin metal ‘fingers’ are moved to the rear surface aswell, leaving the front with much less metal. This is made possible bydrilling tiny vias to connect the front surface with rear-surfacecontacts. With MWT, this requires about 8-200 holes per wafer.

A schematic representation of part of a prior art conventional bulkcrystalline silicon solar cell is depicted in FIG. 1. See Kerschaver etal. “Back-contact Solar Cells: A Review” Prog. Photovolt: Res. Appl.2006; 14: 107-123. The silicon base 101 is the main part of themechanical structure. The emitter 102 is located near the top or frontsurface. A metal grid 103, 104 to extract the carriers from the devicecontacts each of these silicon regions. Whereas the rear surface isoften fully covered by a base contact 105 (as in the drawing), on thefront surface the metal grid is the result of a trade-off between havinglow coverage to limit optical losses and high coverage to limitresistive losses. Most manufacturers apply a front grid consisting ofthin parallel lines 103 (fingers) that transport the current tocentrally located busbars 104. The busbars are relatively wide and canbe used as solder pads for connecting to external leads.

The contact wrap-through or metallization wrap-through (MWT)back-contact cell is the concept that is most closely linked to theconventional cell structure. In these cells, the emitter is located nearthe front surface, but part of the front metallization grid is movedfrom the front to the rear surface. In the schematic representation ofthe prior art cell in FIG. 2, this is depicted as the busbar 104 movingfrom one surface to the other. The remaining front surface grid 103 isconnected to the interconnection pads 107 on the rear surface byextending it through a number of openings 106 in the wafer. The basecontact 105 is electrically isolated from the interconnection pads 107as shown in FIG. 2. See Kerschaver et al. “Back-contact Solar Cells: AReview” Prog. Photovolt: Res. Appl. 2006; 14: 107-123.

The MWT cells provide advantages by moving the front bus bar to theback—the shading losses are minimized, with a resulting increase in cellefficiency. However, in addition to performance optimization, there is aneed for new back-contact cell/module designs that can make use of newassembly technologies that are inherently more scaleable (i.e., largerand/or thinner cells) with improved cost/throughput compared to currentassembly processes using conventional cells. In particular, there is aneed for new MWT cell designs and fabrication methods that arecompatible with thin epitaxial silicon solar cells.

SUMMARY OF THE INVENTION

The present invention includes metal wrap through (MWT) devicestructures and methods for fabricating said structures, which are wellsuited to thin solar cells. The planar back contact geometry of base andemitter contacts in these MWT devices simplifies process flows andassembly methods, thereby reducing cell handling and breakage duringstringing and tabbing, as compared with stringing and tabbing practicedfor conventional front-to-back contact geometries. Although the presentinvention is described with examples of thin film single crystalepitaxial solar cell fabrication, the processes of the present inventionmay be integrated with other solar cell designs and fabrication methods,for example conventional crystalline silicon solar cells,heterostructure solar cells or multi junction solar cells.

According to aspects of the present invention, fabrication of a metalwrap through solar cell may comprise: forming blind vias in the frontsurface of a base layer; forming an emitter on the front surface of thebase layer; depositing an antireflective coating over the emitter;filling the blind vias with electrically conductive material; depositingbusbars over the front surface of the base layer, the busbars beingconfigured to connect to the filled blind vias; attaching the frontsurface of the processed base layer to solar glass using an encapsulant;forming vias from the back surface of the base layer through the baselayer, the vias terminating in the filled blind vias; depositing aconformal dielectric film over the surface of the vias and the backsurface of the base; removing portions of the conformal dielectric filmfrom the ends of the vias for exposing the filled blind vias and fromfield areas of the base; and forming separate electrical contacts to thefilled blind vias and the field areas, wherein the separate electricalcontacts are all accessible on the back surface of the solar cell. Thesolar cells may comprise epitaxially deposited silicon and may includean epitaxially deposited back surface field.

According to further aspects of the present invention, fabrication of ametal wrap through solar cell may comprise: providing a photovoltaicdevice attached at the emitter side of the device to a solar glass by anencapsulant, the device including busbars on the device emitter; formingvias through the device base and emitter, the vias terminating in thebusbars; depositing a conformal dielectric film over the surface of thevias and the back surface of the base; removing portions of theconformal dielectric film from the ends of the vias for exposing thebusbars and from field areas of the base; and forming separateelectrical contacts to the busbars and the field areas on the backsurface of the solar cell. The solar cells may comprise epitaxiallydeposited silicon and may include an epitaxially deposited back surfacefield.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a schematic representation of a conventional solar cell;

FIG. 2 is a schematic representation of a metallization wrap-through(MWT) solar cell;

FIG. 3A is a top view of a representation of the fabrication of a MWTsolar cell showing the cell after top via formation, according to afirst embodiment of the present invention;

FIG. 3B is a cross-sectional view of the MWT solar cell of FIG. 3A;

FIG. 4 is a cross-sectional view of the MWT solar cell of FIG. 3B afterfurther steps of via filling and busbar formation, according to someembodiments of the present invention;

FIG. 5 is a cross-sectional view of the MWT solar cell of FIG. 4 afterfurther steps of encapsulation and attachment to frontside glass,according to some embodiments of the present invention;

FIG. 6 is a cross-sectional view of a representation of the fabricationof a solar cell module comprised of MWT solar cells as in FIG. 5,according to some embodiments of the present invention;

FIG. 7 is a cross-sectional view of the MWT solar cell of FIG. 6 afterthe further step of backside via formation, according to someembodiments of the present invention;

FIG. 8 is a cross-sectional view of the MWT solar cell of FIG. 7 afterthe further step of backside nitride deposition, according to someembodiments of the present invention;

FIG. 9 is a cross-sectional view of the MWT solar cell of FIG. 8 afterthe further step of dielectric opening formation, according to someembodiments of the present invention;

FIG. 10A is a cross-sectional view of the MWT solar cell of FIG. 9 afterthe further step of backside via filling, according to some embodimentsof the present invention;

FIG. 10B is a top view of the backside of the MWT solar cell of FIG.10A;

FIG. 11 is a top view of the MWT solar cell of FIG. 10B after thefurther step of tab attachment, according to some embodiments of thepresent invention;

FIG. 12 is a cross-sectional view of a representation of the fabricationof a MWT solar cell showing the cell after frontside processing,according to a second embodiment of the present invention;

FIG. 13 is a cross-sectional view of the MWT solar cell of FIG. 12 afterthe further step of backside via formation, according to someembodiments of the present invention;

FIG. 14 is a cross-sectional view of the MWT solar cell of FIG. 13 afterthe further step of backside conformal dielectric deposition, accordingto some embodiments of the present invention;

FIG. 15 is a cross-sectional view of the MWT solar cell of FIG. 14 afterthe further step of removal of the dielectric from the bottom of thevias, according to some embodiments of the present invention;

FIG. 16 is a cross-sectional view of the MWT solar cell of FIG. 15 afterthe further step of backside metallization according to a first scheme,according to some embodiments of the present invention; and

FIG. 17 is a cross-sectional view of the MWT solar cell of FIG. 16 afterthe further step of backside metallization according to a second scheme,according to some embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of some embodiments of the invention so as to enable thoseskilled in the art to practice the invention. Notably, the figures andexamples below are not meant to limit the scope of the present inventionto a single embodiment, but other embodiments are possible by way ofinterchange of some or all of the described or illustrated elements.Moreover, where certain elements of the present invention can bepartially or fully implemented using known components, only thoseportions of such known components that are necessary for anunderstanding of the present invention will be described, and detaileddescriptions of other portions of such known components will be omittedso as not to obscure the invention. In the present specification, anembodiment showing a singular component should not be consideredlimiting; rather, the invention is intended to encompass otherembodiments including a plurality of the same component, and vice-versa,unless explicitly stated otherwise herein. Moreover, applicants do notintend for any term in the specification or claims to be ascribed anuncommon or special meaning unless explicitly set forth as such.Further, the present invention encompasses present and future knownequivalents to the known components referred to herein by way ofillustration.

The present invention is described herein with reference to twoembodiments which are designed to be used with thin film single crystalepitaxial solar cell fabrication processes such as described in U.S.patent application publication nos. 2010/0108134, 2010/0108130,2009/0227063 and 2011/0056532, all incorporated by reference herein.However, the present invention is not intended to be limited to theseparticular embodiments, but may be integrated with other solar celldesigns and fabrication methods, for example conventional crystallinesilicon solar cells, heterostructure solar cells or multi-junction solarcells. The first embodiment of the MWT structure and method offabrication are schematically shown in FIGS. 3A, 3B, and 4-11.

FIGS. 3A, 3B, and 4-5 show top and cross-sectional views of a siliconsubstrate with a porous silicon separation layer and epitaxial siliconlayers grown over the porous silicon separation layer. Thecross-sections of FIGS. 3B, 4 and 5 are along X-X as indicated in FIG.3A. Further details of fabrication methods for the separation layer andepitaxial layers are provided in U.S. patent application publicationnos. 2010/0108134, 2010/0108130, 2009/0227063 and 2011/0056532, allincorporated by reference herein, for example.

FIG. 3A shows a top view of an array of blind vias 301 which have beenformed in the front side 302 of the thin epitaxial silicon films aftertexture etching of the surface. A silicon nitride antireflection coating(ARC) 303, an n⁺⁺ emitter 304, an epitaxial silicon p-type silicon base305 (which may be very thin—less than 50 microns thick) and p⁺ epitaxialsilicon back side field (BSF) 306 are shown formed on a porous siliconseparation layer 307 on a silicon substrate 308. For ease ofillustration only 8 vias are illustrated; however, for a typical solarcell there will be in the range of 8 to 200 vias. The number anddiameter of the vias can be optimized for a chosen solar cell size,efficiency and expected current-carrying requirements such that theseries resistance due to the vias is minimized. Photovoltech (a solarcompany headquartered in Belgium) has demonstrated that 156 mmmulticrystalline solar cells with cell efficiency of approximately 16%,with the ability to operate the cell at 7.8 A and approximately 0.5 V(current and voltages represent the value at maximum power point) withonly 16 vias of 125 microns. In this invention, the vias areapproximately 30-50 microns deep and 0.2-0.3 mm in diameter, but thesedimensions may be adjusted to optimize the cell performance parameters.The vias may be formed by laser drilling, although other drillingmethods, such as, plasma etching, ion beam, directed water or abrasivejets, EDM (electric discharge machining) may be used. After viaformation, diffusion of the emitter is carried out, typically in a tubefurnace or inline system. The emitter of a conventional p-type PV cellis typically diffused by flowing POCl₃ in a furnace at an elevatedtemperature (between 750-900° C.) for a pre-determined time or byspraying a phosphorous containing acid followed by drive-in of thedopant species (phosphorous, in this example) at an elevated temperature(between 750-900° C.) for a pre-determined time. The diffusion isfollowed by a silicon nitride deposition over the surface of theepitaxial layers and the surface of the blind vias; the silicon nitrideforms an ARC. See the cross-sectional representation in FIG. 3B. Thenitride for solar applications is typically deposited by plasma-enhancedchemical vapor deposition (PECVD) or by reactive sputtering. The nitrideis typically ˜70-100 nm thick with a refractive index close to 2 toprovide good anti-reflection performance. Typically silane, hydrogen andammonia gas mixtures are employed in CVD reactors to provide the optimalproperties for the silicon nitride, and in reactive sputtering, siliconis sputtered in an ammonia, hydrogen mixture at low pressure.Consequently, there is hydrogen available to passivate the danglingbonds on the surface of the silicon emitter. Furthermore, note that thisnitride is an insulator and will provide an insulating barrier betweenthe emitter contact and the base p-type silicon absorber.

FIG. 4 shows a silver metal lug 309 filling the blind via and bus bar310 formed by screen printing Ag paste. The busbars 310 runperpendicular to the plane of the figure and may be configured asfingers, grids, etc. (Note that herein the term busbar is used to refergenerally to busbars, collecting fingers, collecting grids, etc.)Roughly 2-60 microns of Ag is deposited, so that the Ag metal lugsprotrude from the surface of the epitaxial layers. This can beaccomplished by screen printing the Ag paste in one or multiple printingsteps. Commercial Ag pastes that usually have the capability to “firethrough” nitride during the firing cycle may be used. Hence, during thefiring of the screen printed silver, the Ag fires through the nitrideand forms an ohmic contact with the emitter surface in the blind viasand under the busbars.

Next, the silicon wafers are mounted on glass and exfoliated usingtechniques described in U.S. patent application publication nos.2010/0108134, 2010/0108130, 2009/0227063 and 2011/0056532, allincorporated by reference herein, for example. FIG. 5 shows a singlecell attached by encapsulant 311 to solar PV glass 312, prior toexfoliation from the substrate 308. The encapsulant 311 may be roughly200 microns thick and the solar glass 312 may be approximately 3 mmthick.

FIG. 6 shows, in schematic cross-section, multiple cells attached by anencapsulant 311 to a solar PV glass substrate 312, where all of theepitaxial Si cells have been exfoliated from their Si substrates. Thesilver busbars 310 and epitaxial silicon layers 320 are shown for eachcell. For ease of illustration, the ARC, emitter layers, p-base and BSF(back surface field) are nor explicitly shown, also only three wafersare shown attached to the solar glass. In actual practice, amultiplicity of cells will be attached at the same time to the solarglass and exfoliated. For example typical modules have 72-96 cells, andthe said number of cells will be attached to the glass. Note that hereinthe term solar glass is used to refer to front sheet materials withsuitable optical transparency, mechanical and handling properties;examples of front sheet materials include suitable glasses, polymers andtransparent ceramics.

FIG. 7 shows vias 313 which have been opened up on the backside toconnect with the front side busbars using a rough alignment process(simple registration might be sufficient given the hole sizerequirements and the need for the hole to be completely captured by thebusbar). The vias are roughly 300-500 microns in diameter and deepenough to expose the front side busbars (roughly 30 microns deep). Thesevias may be laser drilled as described above, or other processes may beused, again as described above. A wet clean used for healing laserdamage can be done at this time—care being taken in the choice ofcleaning chemistry to prevent damage to the solar, glass, metal orencapsulant.

FIG. 8 shows a conformal coating of silicon nitride dielectric 314(alternatively, other dielectrics such as, silicon oxide dielectric maybe used) covering the backside of the epitaxial layers and the surfaceof the drilled backside vias. The dielectric liner for the backside viasis provided by the same blanket dielectric layer used for optical light(IR) confinement on the backside.

FIG. 9 shows laser drilled openings 315 formed in the dielectric layer314 at the frontside busbars; laser drilling is also used at this stepto expose the p⁺⁺ layer in field regions to allow formation of backcontacts to the base. The vias may be formed by laser drilling, althoughother drilling methods, such as, plasma etching, ion beam, directedwater or abrasive jets, EDM (electric discharge machining) may be used.

Next, aluminum (often deposited as a sandwich comprising Al and V, toprevent oxidation and allow solderability) stripes are deposited overthe field regions while the frontside busbars are protected by a shadowmask. This is followed by depositing conducting paste (silver-filledepoxy epoxy or a low temperature colloidal silver paste) in the backsidevias using a syringe-type dispense technique or a screen printingprocess. See FIG. 10A. The same conducting paste is also deposited onthe aluminum stripes at the position of the back contacts to the p⁺⁺layer, for making the P tabs. The curing temperature of the silver pasteshould be sufficiently low such that the module encapsulant (thatattached the thin cells to solar glass) is not degraded. FIG. 10B is atop view of the backside of the three solar cells showing back contactstripes 317 and filled backside vias 316 contacting the front of thecells.

FIG. 11 shows a top view of the same three wafers of FIG. 10B, showingthe frontside and backside silver contacts, and the way in which theseparate solar cells are connected together using straight line tabs318. After straight line tabs are applied, there is a final laminationwith ethylene vinyl acetate (EVA) encapsulant and a Tedlar® polyvinylfluoride (PVF) backsheet. (The latter lamination process may also beeffective in curing the conducting paste.)

The second embodiment for this unique MWT approach is shown in schematiccross-section in FIGS. 12-17. This embodiment does not require the frontside vias but may have less process margin than the first embodiment dueto the need to drill the backside vias completely and yet stop on Agwhile accounting for thickness variation of the epitaxial siliconlayers. Conversely, the second embodiment may have a higher processmargin and lower cost if the laser drilling (and other methods fordrilling vias) allows for a highly selective way to stop on the Agbusbar after drilling through the silicon, due, for example, to a verylarge difference in the drilling rate of the silicon and metal—thedrilling rate of the metal being the lesser. Note that thecross-sections of FIGS. 12-17 are perpendicular to the cross-sections ofFIGS. 3B, 4-9 and 10A-FIG. 10B shows the cross-sectional plane Z-Z forFIGS. 12-17. (The structures of the first and second embodiments aredifferent; however, the basic layout of the contacts is the same—asshown in FIGS. 10B and 11.) The cross-sections of FIGS. 12-17 show bothfrontside and backside contacts for a single cell.

FIG. 12 shows an epitaxial silicon thin film structure which has beenexfoliated from the silicon substrate on which the epitaxial layers weregrown. The structure of FIG. 12 may be fabricated using processes suchas described in U.S. patent application publication nos. 2010/0108134,2010/0108130, 2009/0227063 and 2011/0056532, all incorporated byreference herein. The epitaxial silicon thin film structure is attachedto a solar glass 507 with an encapsulant 508 prior to separation of theepitaxial layers from the silicon substrate; ordinarily multipleepitaxial silicon thin film structures are attached to a single sheet ofsolar glass to make a solar module as described above. Frontside metalgrid lines 506 (and/or portion of the busbar) were deposited prior toseparation from the silicon substrate; methods such as screen printingmay be used to deposit the grid lines 506. The silicon epitaxial layersare roughly in the range of 20-50 microns thick and include a p⁺ BSF(back surface field) layer 501 and a p⁻ layer 502. The p⁺ BSF (backsurface field) epitaxial layer 501 is on the backside and is covered bythe remains of the porous silicon separation layer 503. There is an n⁺emitter 504 covered by a silicon nitride ARC/passivation layer 505 onthe frontside.

In FIG. 13, laser vias 511 are drilled from the back of the exfoliatedthin epitaxial silicon. The laser via must be fabricated such that thevia is terminated at the front metal gridline or pad 506 with minimalloss of metal.

The entire backside is now cleaned up to remove the remaining poroussilicon and any laser damaged regions—this may be done in one wetchemical step, such as a hot KOH etch. A thin conformal dielectric layer512, such as SiN or sputtered quartz or undoped amorphous silicon, isthen deposited on the entire backside with certain areas protected by ashadow mask during deposition. The dielectric acts as a passivationlayer and optical confinement enhancer for the backside p⁺ layer 501,and also acts as an insulator in the via—separating the hole currentfrom the electron current. See FIG. 14.

The dielectric is then selectively removed from the areas 513 in the viaregion where the SiN is contacting the frontside metal by a second“touchup” laser ablation to enable an electrical contact to the frontgridline/pad 506. See FIG. 15.

The backside surface is metalized, with Al metallization for example, inone of two ways:

By the first method (FIG. 16), aluminum 514 is deposited in the areasthat contact the base (in the regions masked during SiN deposition). Thelaser vias are then filled with Ag-filled epoxy 515 in a separate step,using a syringe-type dispense technique or a screen printing process,for example. The Ag-filled epoxy may also be applied to the Al layerbase contacts to assist with attachment to the tabbed interconnectionsbetween cells.

The second, alternate, method uses a blanket Al (or AlN sandwich) 601deposition by a physical vapor deposition method such as sputtering ore-beam evaporation to fill the vias and form base contactssimultaneously. The deposition should be done in such a manner so as tocreate isolation between the emitter and base metallization either by asuitable shadow mask employed during the Al deposition, or selectiveremoval of metal by laser to create insulation from the front andbackside metallizations. See FIG. 17.

Cell to cell interconnects may be established as in the first embodimentand depicted in FIG. 11.

All back Aluminum co-fired contacts are typically employed in siliconsolar cells since they serve the dual purpose of creating an Al-dopedp⁺⁺ back surface field (BSF), in addition to improving back contacts tothe base. However, this method does not result in an optimal BSFstructure and may induce a bow in thin silicon wafers. A single crystalsilicon solar cell with an insitu p⁺⁺ BSF with p-type epitaxial siliconwill obviate the need for the conventional Al screen printing step, thusenabling a thinner silicon solar cell since high temperature Al firingis eliminated and thereby issues relating to bowing and attendantstresses are also eliminated. For example, as part of the epitaxialsilicon deposition an epitaxial film of p⁺⁺ silicon BSF (resistivity of1-3 mohm-cm) may be deposited on the annealed surface of the poroussilicon layer, approximately 1-10 microns thick, in the epitaxialdeposition reactor, followed by epitaxial deposition of the base on topof the BSF. The epitaxial solar cell design including epitaxiallydeposited BSF is important since it eliminates the need for an all metalback contact, instead of using the p⁺⁺ layer to contact a metal grid(where the grid may have 5% surface coverage) or other configurations ofmetal point contacts, for example. The metal grid may be formed from,for example: Al/Ag pastes, fits, or epoxies; plated Ni and/or platedTi/Pd/Ag. The epitaxial cell of the present invention with theepitaxially deposited BSF may include the following advantages over aconventional cell: lower cell manufacturing cost since full Al backcontact screen printing is avoided and thinner silicon is enabled.

Furthermore, the methods of the present invention for fabricating MWTcontacts may be employed with minor changes to be applicable to n-typesolar cells, as well as to other device architectures such as rearsideemitter with or without front surface field (FSF).

Embodiments of the present invention may provide one or more of thefollowing advantages. Embodiments of the present invention utilize theplanar back contact geometry of base and emitter contacts to simplifyprocess flows and assembly methods, thereby reducing cell handling andbreakage during stringing and tabbing, as compared with stringing andtabbing practiced for conventional front-to-back contact geometries—thisfacilitates the application of metal wrap through technology to a widerrange of structures, such as thin silicon. Some embodiments of thepresent invention allow sequential access to the front and backside ofsolar cells with passivated base and local back contacts, facilitatinghigh cell efficiencies at high frequencies. Some embodiments of thepresent invention include processes with front and backside partialvias, which may increase process margins. Some embodiments of thepresent invention require only a single screen printing (as opposed to 3in the conventional MWT process), followed by a low-cost, large formatAl deposition by evaporation or sputtering. Some embodiments of thepresent invention are processes that may be employed with a singledielectric layer on the backside for both optical confinement andpassivation; furthermore, the same dielectric layer may be utilized forthe via liner.

Although specific examples have been provided of filling vias with Agfilled epoxy and applying Ag filled epoxy to base contacts, a wide rangeof conductive pastes may be used for these purposes in the presentinvention.

Although the present invention has been described with reference toembodiments which include an epitaxially deposited BSF, the principlesand concepts of the present invention may be applied to solar cellswithout a BSF, solar cells without an epitaxially deposited BSF, andsolar cells with a BSF formed by other methods. For example a BSF may beformed by ion implantation of the base, or by other suitable methods.

Although the present invention has been particularly described withreference to certain embodiments thereof, it should be readily apparentto those of ordinary skill in the art that changes and modifications inthe form and details may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of fabricating a metal wrap throughsolar cell comprising: forming blind vias in the front surface of a baselayer; forming an emitter on the front surface of said base layer;depositing an antireflective coating over said emitter; filling saidblind vias with electrically conductive material; depositing busbarsover the front surface of said base layer, said busbars being configuredto connect to said filled blind vias; attaching the front surface ofsaid processed base layer to solar glass using an encapsulant; formingvias from the back surface of said base layer through said base layer,said vias terminating in said filled blind vias; depositing a conformaldielectric film over the surface of said vias and the back surface ofsaid base; removing portions of said conformal dielectric film from theends of said vias for exposing said filled blind vias and from fieldareas of said base; and forming separate electrical contacts to saidfilled blind vias and said field areas, wherein said separate electricalcontacts are all accessible on the back surface of said solar cell. 2.The method of claim 1, wherein said base layer is epitaxial silicon. 3.The method of claim 2, wherein said base layer is less than 50 micronsthick.
 4. The method of claim 2, wherein said base layer is attached toa porous silicon separation layer during said forming blind vias, saidforming an emitter, said depositing an anti-reflective coating, saidfilling said blind vias, said depositing busbars, and said attaching,and wherein said porous silicon separation layer is formed on thesurface of a silicon substrate.
 5. The method of claim 4, furthercomprising, after said attaching, separating said processed base layerfrom said silicon substrate.
 6. The method of claim 1, wherein saidforming separate electrical contacts includes depositing conductingmaterial in said vias and on said field areas of said base.
 7. Themethod of claim 1, wherein said base layer includes an epitaxiallydeposited back surface field on the back surface of said photovoltaicdevice.
 8. The method of claim 7, wherein said forming separateelectrical contacts includes forming a metal grid on said field areas.9. The method of claim 1, wherein said attaching process is theattaching in an array of a multiplicity of processed base layers to saidsolar glass.
 10. A method of fabricating a metal wrap through solar cellcomprising: providing a structure including a photovoltaic deviceattached to a solar glass by an encapsulant, said photovoltaic deviceincluding a base, an emitter formed on the front surface of said base,an anti-reflective coating on said emitter and busbars on said emitter,wherein said busbars and antireflective coating are on the front surfaceof the photovoltaic device and face said solar glass; forming viasthrough said base and said emitter, said vias terminating in saidbusbars; depositing a conformal dielectric film over the surface of saidvias and the back surface of said base; removing portions of saidconformal dielectric film from the ends of said vias for exposing thebusbars and from field areas of said base; and forming separateelectrical contacts to said busbars and said field areas, wherein saidseparate electrical contacts are all accessible on the back surface ofsaid solar cell.
 11. The method of claim 10, wherein said formingseparate electrical contacts includes depositing conducting material insaid vias and on said field areas of said base.
 12. The method of claim10, wherein said base and said emitter are epitaxially depositedsilicon.
 13. The method of claim 12, wherein said base layer is lessthan 50 microns thick.
 14. The method of claim 12, wherein saidphotovoltaic device further includes an epitaxially deposited backsurface field on the back surface of said photovoltaic device.
 15. Themethod of claim 14, wherein said forming separate electrical contactsincludes forming a metal grid on said field areas.